Arc melting crucible



May 27, 1958 R. J. KRIEGER ARC MELTING CRUCIBLE Filed Aug. 22, 1955 M m .g a J A, ,/A 1 A ...q 2 wm 6 e u M 6;\\\\\\\\\\\\\\\ may i O 6 2 I;I FW 4.1 4 o 4 1 3 Af r .i 2 2 Z e .LL R W V M\% i I IAlIIHI M u A. a |ll @W Si mv\ |l\ l" ww\ FI @u .IWZ Mw mw United States Patent O ARC MEL'IIN CRUCIBLE Robert 3. Krieger, Las Vegas, Nev., assigner to Titanium Metals Corporation of America, Pittsburgh, Pa., a corporation of Pennsylvania Application August 22, 1955, Serial No. 529,629

4 Claims. (Cl. 2645-39) This invention relates to crucibles employed for melting metals in arc furnaces, and more particularly to crucibles which are cooled and known as the cold mold type.

Crucibles for containing metal melted to form ingots or cast shapes in an arc furnace have heretofore consisted of a shell, preferably of copper, surrounded by a water jacket. Such construction permits the melting of high melting point metals, such as titanium or zirconium, employing a Consumable or non-consumable electrode, even though the Crucible material may be of lower melting point than the metal being melted. A typical furnace design is shown in Southern Patent No. 2,686,825 issued August 17, 1954. Since the Crucible is connected to one pole of the electric current supply and the electrode another, there is an ever present danger, in operation of such furnaces, of short Circuiting and/or arcing from the electrode to the Crucible wall instead of to the molten metal in the Crucible. This may occur because of misalignment of the electrode, or deposition of metal particles on the Crucible wall which decreases the electrode clearance, or other causes. When shorting or arcing to the Crucible wall occurs, the heat of the arc quickly melts a hole in the metal shell and water from the cooling jacket enters the melting zone and contacts the molten metal. The results may vary from a mild evolution of steam to a catastrophic explosion which may completely destroy the furnace as Well as injuring and killing operating personnel. ln any event the ingot is almost invariably ruined due to oxidation at the high temperature involved, and in the case of large ingots of valuable metals, this may represent a considerable loss. In the case of arc furnaces operating under vacuum the danger is much increased due to the greater ditiiculty in controlling the arc, and due to the pressure differential a larger volume of water quickly pours in onto the molten metal when perforation of the Crucible shell wall occurs.

This invention has for its object provision of an improved Crucible for metals melted in an arc furnace. Another object of this invention is to provide a safer crucible for metals melted in an arc furnace. A further object is to provide a Crucible for arc melting furnaces which will prevent cooling liquid from entering the melting zone if arcing to and perforation of the Crucible wall occurs. These and other objects of this invention will be apparent from the following description thereof and from the annexed drawings in which:

Fig. l shows a sectional side view of the Crucible of this invention. v

Fig. 2 is a horizontal section of the Crucible of Fig. l taken along the line 2 2.

Fig. 3 shows in more detail the construction of the inner shell wall of the Crucible of Fig. l.

Referring now particularly to Figs. l and 2, a Crucible embodying feature of thisk invention comprises au outer shell i@ which may be of generally tubular cross section closed at the bottom by plate l2. The top of 2,836,412 Patented May 27, 158

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shell 1l) is conveniently provided with out-turned ange 14 to which the bottom of the furnace proper, indicated generally at 16, may be attached as by bolts 18. The inner shell 20 of the Crucible which actually contains the metal during melting is fabricated of relatively thick stock and also may be of tubular cross section and arranged concentrically with respect to outer shell 10. The bottom of shell 28 is closed by plate 22. The inner Crucible shell 2li is characterized by Containing internal inert gas containing meansintegral therewith, and which underlies a portion of the inner surface thereof. ln the embodiment described, this is accomplished by provision of an internal helical duct 24. The duct may be conveniently fabricated, as shown more clearly in Fig. 3, by machining a spiral groove inthe outer surface of a tubular piece of metal, preferably copper, the sides and bottom of the groove forming similar elements of the duct. A copper sheet 26 is then overlaid over the wall and brazed to the lands 28 between the grooves previously machined. Suitable Connections 30 and 32 are made respectively to the'bottom and top end of the duct, and these pass through outer shell 10 and are connected to conventional means for supplying inert gas to the duct 24 through which it is preferably circulated. The top of inner shell 20 is provided with out-turned llange 34, which is brazed or welded to outer shell lu so as to form around inner shell 20 liquid cooling jacket 35. Inlet 38 and outlet 40 are provided to enable Cooling liquid, such as water, to be circulated through jacket 36.

ln operation during melting, a suitable inert gas, which may be argon or helium, is circulated through duct 24 and a cooling liquid, conveniently water, is circulated through cooling jacket 36. The area of the inner surface of the wall of shell 20 will be somewhat warmer in the vicinity of the underlying duct 24 than that adjacent the solid metal which separates the duct loops. This occurs because the heat transfer to the cooling uid will be greater through the solid metal sections than through those where the inert gas intervenes. An arc, as is well-known, will selectively short circuit to a hotter area rather than a cooler, so that any accidental arcing to the Crucible wall will occur in the area over the inert gas duct. The duct 24 being helical and underlying the inner surface of shell 20 forms, in any particular portion of the shell surface, a series of spaced apart areas, alternately cooler and hotter, and arcing to the wall will preferentially occur to the hotter area rather than the cooler. Therefore, if perforation occurs, the duct will be opened to the Crucible interior and only the inert gas will enter the melting zone with no danger of explosion or adverse eect on the metal being melted.

Helium is advantageous to employ as the inert gas in the Crucible or" this invention, especially when the melting operation is carried out under lower than atmospheric pressure. A vacuum gage of the ionization type employing alpha particles as an ionization source is very sensitive to helium. Therefore, employment of such a gage will provide an instant indication of the presence of helium in the furnace atmosphere, indicating a leak in the inner shell wall, and the furnace may be shut down without delay avoiding possible product loss and danger to personnel.

ln operation of the Crucible of this invention, suiiicient cooling liquid, advantageously water, is passed through the cooling jacket to keep the inner Crucible shell at a temperature generally aboutp400 F. or below. Helium or other inert gas, which will not contaminate the metal being melted or Cause explosive vconditions in Contact with themolten metal, is circulated through the duct in the inner Crucible shell at a convenient rate, which may `vary considerably depending largely on the size and op- ,erating characteristics of the furnace. ltlmay be desira'ble to circulate inert gas Ythrough the duct at a suil cient velocity `to abstract an appreciable amountV of heat,

and under these conditions avsirnple and conventional gas cooling arrangement may be provided and the inert gas recyclediV Y y Y l claim; a

1. A cruciblejfor containing metals being'meltedY in an are furnace comprising; an inner shell, inert gas containing means adapted to underlie Aa portion of the inside' surfac'eo'f said inner shell, an outer shell surrounding said innerl shell, and liquid cooling means between said innerand outer shells to cool said inner shell. Y

2.V A Crucible for containing'me'tals being melted in an arc furnace comprising; an inner-shell, said inner. shell having an internal integral'druct containing an inert gas nnderlyinga `portiorfof the inside surface thereof, an outer shell surrounding said inner shell, and liquid cooling means between said inner and outershells Yto cool said inner shell.'Y Y Y Y 3'. VA Crucible for containing metals being melted in an arc furnace comprising; an Yinner shell, said inner shell having an internal integral helical duct therein containing Y an inert gas, an outer shell'surrounding said inner shell,

` and liquid cooling meansbetween said inner and outer shells to cool said inner shell'.

4. A Crucible for containing metals being melted'in an `arc furnace comprising; aninner shell, said inner shell having an internal duct containing an inert gasunderlying `spaced apart areas of the inside surface thereof, an outer Y Vshell surrounding said inner shell, and liquid cooling means between said inner and outer'shells to cool said inner shell.

ReferencesnCited in the le of this patent,Y

Y UNTED STATES PATENTS Terhune Aug, `4, 1885kr Great Britain Apr.Y 25, 19,56 

1. A CRUCIBLE FOR CONTAINING METALS BEING MELTED IN AN ARC FURNACE COMPRISING; AN INNER SHELL, INERT GAS CONTAINING MEANS ADAPTED TO UNDERLIE A PORTION OF THE INSIDE SURFACE OF SAID INNER SHELL, AN OUTER SHELL SURROUNDING 